2021 Apple TV 4k and 2021 Samsung's Neo QLED QN90A and SONOS Arc soundbar issues?

Set your Apple TV to output the audio to TV speakers, then let your TV pass that through to the soundbar. That is the wired connection that will work best. Don’t use wireless for TV audio where A/V sync is paramount. Use HDMI instead.

If you set Apple TV to output the audio to the soundbar, then that will be wireless (AirPlay or Bluetooth), which could introduce its own issues. You would also have to set that each time, which is a bit of a hassle. Avoid that if possible.

Use the eARC HDMI port on the TV for the connection to the soundbar. See if the eARC HDMI port on your TV needs configuration for high bandwidth audio to the soundbar.

Use HDMI cables rated 18 Gbps or better. Use HDMI cable rated 48 Gbps for the connection between the Apple TV and the TV (the latter is for video bandwidth mainly, but still advisable to ensure that the cable is never the limiting factor for devices with HDMI 2.1 ports).

So I have a related question about setup. I have the latest Apple TV 4k for video and Atmos. My TV (LG OLED 55G1PUA) had 1 HDMI eARC port and I play sound thru a Sonos Arc sound bar (which has HDMI eARC). So in order to get 4K to the TV and Dolby Atmos to the sound bar, do I connect a 48Gbps HDMI cable from the Apple TV to a regular HDMI input to the LG TV then run a 48 Gbps HDMI cable from the LG HDMI eARC port to the Sonos Arc HDMI eARC port?

That will work.

Apple TV 4K (2nd gen) →[HDMI 48 Gbps]→ TV with HDMI 2.1 →[HDMI eARC]→ soundbar

The TV-soundbar connection doesn’t require 48 Gbps cables (it will work fine, but just 0.037 Gbps will be useable by eARC.